Reply by unbob

Often a #5 will do a much better job then a #8.A #5 likely has much less twist and dips in the sole then a much longer #8.The longer the plane the more its bound to be untrue. Each swipe induces the planes as is condition error to the board.A very hard problem to deal with even for a well equipped machine shop. The planes are thin castings, and hard to fixure or clamp into milling machines, or surface grinding machines, without distortion from just that step.Sanding the planes sole is a very poor method-often called lapping in woodworking circles.